Friday, 1 May 2009

Like many people with autism, the celebrated artist Stephen Wiltshire has an incredible ability to focus on small details, as evidenced by his beautifully intricate art-work (see image). However, in the lab, psychologists have struggled to pin down this feature of autism.

Some studies have revealed a global-processing deficit, some haven't. Others have shown a local-processing bias, some haven't. The very latest findings suggest that some, but not all, children with autism specifically show an exaggerated difficulty switching from the local detailed level to a more big-picture global level: an anomaly that can actually lead to advantages when attention is left focused on tiny details. Now a new study has linked this attentional style with head size. It's an exciting finding that could help explain why not all children with autism show the attentional anomaly, and which could also help link the cognitive anomaly with a neurological mechanism.

Sarah White and colleagues tested 49 high-functioning children with autism and 29 neurotypical controls on a task that required them to flick their attention back and forth from a local to a more global level. Specifically they had to either identify large letters made up of smaller blocks, or they had to identify lots of smaller letters that were the size of those blocks.

Consistent with recent findings, a portion of the children with autism showed a very specific deficit - that is, their performance was poorer than the other children when switching from processing at the local to the global level. Crucially, it was the autistic children with abnormally big heads who were the ones to show this anomaly.

A possible neural mechanism underlying this local-global switching deficit seen in some autistic children is abnormal brain wiring, perhaps originating during the pruning phase of neural development when many neurons and synapses are destroyed in a carefully controlled biological process. The new findings suggest that an enlarged head could be a marker for the existence of this abnormal wiring.

A follow-up study tested 12 neurotypical children with big heads and found that, unlike autistic children with big heads, they did not show a deficit in switching from local to global processing. However, these neurotypical children were physically larger in height as well as head size, unlike the big-headed autistic children who were the same height as their smaller-headed peers. This reinforces the idea that it is only when head size is a marker for abnormal brain wiring - as seen in some children with autism - that it is linked with a cost switching to global processing.

"The tentative hypothesis can therefore be proposed," the researchers said, "that head size may be a biological marker of abnormal neural connectivity, resulting in a locally oriented processing style, and may provide a useful endophenotype for investigating the genetic basis of a subgroup of individuals with autistic spectrum disorder."
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